If the total atmospheric pressure is 380 mmHg and the oxygen fraction is 21%, what is the partial pressure of oxygen?

If the total atmospheric pressure is 380 mmHg and the oxygen fraction is 21%, what is the partial pressure of oxygen?

• A. 80 mmHg
• B. 159.6 mmHg
• C. 300 mmHg
• D. 400 mmHg

Answer: (A)

Partial pressure of a gas in a mixture is its fraction times the total pressure. Oxygen makes up 21% (0.21) of the air, so PO2 = 0.21 × 380 mmHg = 79.8 mmHg, about 80 mmHg. This shows how changes in total atmospheric pressure scale the partial pressures of the constituents. The other numbers would come from using a different total pressure (for example, 0.21 × 760 ≈ 159.6 mmHg) or from misplacing the decimal, but with the given total pressure, the correct value is about 80 mmHg.

Partial pressure of a gas in a mixture is its fraction times the total pressure. Oxygen makes up 21% (0.21) of the air, so PO2 = 0.21 × 380 mmHg = 79.8 mmHg, about 80 mmHg. This shows how changes in total atmospheric pressure scale the partial pressures of the constituents. The other numbers would come from using a different total pressure (for example, 0.21 × 760 ≈ 159.6 mmHg) or from misplacing the decimal, but with the given total pressure, the correct value is about 80 mmHg.